The hydrogenation of oils or fats is carried out to produce a more oxidatively stable product and/or to change a normally liquid oil into a semi-solid or solid fat with characteristics designed for a particular product application. The goal of an oil hydrogenation processing scheme is to reduce the number of unsaturated fatty acids or fatty acid constituents present in the triglycerides.
The majority of commercially hydrogenated oils and fats are processed with batch reactor equipment using high temperatures, chemical catalysts, and hydrogen gas supplied to the reactor at elevated pressures. The hydrogenation catalysts used include Raney and supported nickel catalysts, promoted nickel catalysts containing palladium, copper, or zirconium, and copper chromite catalysts. The rate of hydrogenation is dependent on the reaction temperature, the nature of the oil or fat, the activity and concentration of the catalyst, and the rate at which hydrogen gas and unsaturated oil or fat are supplied to the hydrogenation reactor. Typical reaction pressures and temperatures are in the range of 10-60 psig and 150.degree.-225.degree. C., respectively. These elevated temperatures and pressures are required to solubilize sufficiently high concentrations of hydrogen gas in the oil/catalyst or fat/catalyst reaction medium so that the hydrogenation reaction proceeds at acceptably high rates. Unfortunately, high reaction temperatures promote a number of deleterious side-reactions such as the production of trans fatty acid isomers, the oxidation of double bonds leading to flavor reversion and rancidity, and the formation of cyclic aromatic fatty acids.
In traditional high temperature hydrogenations some of the unsaturated cis isomers of fatty acids or triglycerides are converted to the trans isomers. This transformation may cause the hydrogenated oil or fat to have undesirable properties and may affect the nutritional value of the oil or fat.
For example, as discussed by J. D. Ray et al. in "Empirical Modeling of Soybean Oil Hydrogenation", J. Am. Oil. Chem. Soc., 62, 1222 (1985), the total percent of trans isomers in a partially hydrogenated soybean oil product ranges from 40.8% to 60.8%, whereas the initial oil contains only 3.5% trans isomers. In this study, the temperature was between 138.degree. C. and 204.degree. C., the pressure was between 5 and 50 psig, and the catalyst was nickel. A common measure of quantifying the extent of cis to trans isomer conversion during the hydrogenation of an oil or fat is the specific isomer selectivity index, defined as the percent of trans isomers in the hydrogenated oil product divided by the change in Iodine Value between the starting oil and hydrogenated product. A typical commercial hydrogenated corn oil margarine, for example, has an Iodine Value of approximately 93, whereas the unreacted liquid corn oil starting material has an Iodine value of approximately 128. The total trans isomer content of the hydrogenated corn oil margarine is 20.5%, thus the specific isomer selectivity index is 0.590, i.e., 20.5/(128-93). Hydrogenated oil products from typical high temperature (60.degree.-170.degree. C. ) hydrogenation processes have a specific isomer selectivity index in the range of 0.36 to 1.79, indicating high trans isomer concentrations [see, for example, "Homogeneous Catalytic Hydrogenation of Soybean Oil: Palladium Acetylacetonate", J. Am. Oil. Chem., 62, 517 (1985)].
Although electrochemical reductions of simple unsaturated organic compounds have been widely studied over the past fifty years, very little work has been carried out on the electrochemical reduction of oils or fats. An electrochemical technique for adding hydrogen to an oil is described by L. M. V. Tillekeratne, et al. in "Electrochemical Reduction of Rubber Seed Oil to Stearic Acid", J. Applied Electrochemistry 11, pp. 281-285 (1981). This electrochemical reduction of rubber seed oil via a direct electron transfer mechanism was studied using cathodes such as graphite, copper, stainless steel, lead, nickel, palladium-plated graphite, and Monel (65% nickel and 35% copper). The optimum cathode material was found to be Monel, which is a high hydrogen overvoltage material. No reduction was observed on low hydrogen overvoltage materials such as platinum and nickel.
Electrocatalytic hydrogenations using Raney nickel or similar low hydrogen overvoltage catalysts as cathode materials have been reported by a number of investigators [T. Chiba et al., Bull. Chem. Soc. Jpn., 56 (1983) 719; L. L. Miller et al., J. Org. Chem., 43 (1978) 2059; I. V. Kirilyus et al., Sov. Electrochem., 15 (1979) 1330; K. Park et al., J. Electrochem. Soc., 132, (1985) 1850]. These studies have dealt with the electrochemical hydrogenation of unsaturated hydrocarbons, phenols, ketones, nitro-compounds, and sugars rather than unsaturated fatty acids.
There is a need for a more efficient and alternative method of hydrogenating unsaturated fatty acids and the unsaturated fatty acid constituents in the triglycerides found in oils and fats.